1. Field of the Invention
The present invention relates generally to a hydraulic control device for a continuously variable transmission of belt-and-pulley type, and more particularly to techniques for minimizing a loss of a drive power which is consumed for delivering a pressurized fluid from a hydraulic source to the transmission.
2. Related Art Statement
A continuously variable transmission of belt-and-pulley type is known, which comprises a first and a second variable-diameter pulley provided on a first (input) and a second (output) shaft, respectively, a transmission belt connecting the first and second variable-diameter pulleys, and a first and a second hydraulic cylinder to change an effective diameter of the pulleys. In such a continuously variable transmission, a ratio of a thrust of the second hydraulic cylinder (for the output shaft) to that of the first hydraulic cylinder (for the input shaft) must be changed over a relatively wide range, for example, from 1.5 to 0.5, in order to change a speed ratio of the transmission for providing output speeds over a wide range. FIG. 9 shows an example of a relation between the thrust ratio of the transmission and the speed ratio "e", i.e., the thrust ratio which must be changed according to a change in the speed ratio "e". Reference character P indicates the relation between the thrust ratio and the speed ratio "e" when a positive load torque is applied to the transmission, while reference character N indicates the relation when a negative load torque is applied to the transmission. Further, reference character M indicates the relation when no load is applied.
For such type of continuously variable transmission, there is used a hydraulic control system as disclosed, for example, in Japanese Patent Application which was laid open in 1977 under Publication No. 52-98861. Such a hydraulic control system employs a single line pessure common to the first and second hydraulic cylinders. The line pressure is directly supplied to the second of the two cylinders to maintain an optimum tension of the transmission belt. To the first cylinder, the line pressure is supplied through a flow control valve which is assigned to control a flow of the working fluid fed into the first cylinder or a flow of the fluid discharged from the first cylinder to a drain line, for example, in order to control the speed ratio "e" of the transmission. To change the thrust ratio of the first and second hydraulic cylinders over a wide range, the pressure receiving area of said first cylinder which is controlled by the flow control valve must be about twice as large as that of said second cylinder. In this arrangement, the first cylinder must have a large diameter, which leads to an increased overall size of the transmission. Further, the above arrangement causes an increased moment of inertia of the members on the first or input shaft, and requires a large amount of working fluid to change the speed ratio. Accordingly, the known hydraulic control system as described above fails to provide a satisfactory operating response.
An alternative hydraulic control system is known, which uses two line pressures, i.e., first and second line pressures which are controlled by first and second pressure regulating valves, respectively. The first line pressure which is lower than the second line pressure, is used for the previously indicated second of the two hydraulic cylinders primarily for controlling the tension of the transmission belt. The second line pressure is applied to a flow control valve. In this type of hydraulic control system, a pressure difference between the first and second line pressures assures a relatively large difference in thrust between the two cylinders, even if the two cylinders have substantially the same pressure receiving areas. A hydraulic control system of this type is disclosed in Japanese Patent Application which was published in 1973 under Publication No. 48-26692 for opposition purpose.